1. Field of the Invention
The present invention relates to a method of obtaining land-based seismic data. More particularly, the present invention relates to a method and apparatus for accurately determining the positions of geophones deployed in a geophysical spread.
2. Description of the Prior Art
In conventional land-based seismic studies, individual (analog) seismic sensors called geophones are implanted into the earth generally along a targeted seismic survey line. Each geophone generally has a case that may be buried or coupled to an earth spike for being driven into the earth by applying an inserting force to the top of the geophone case. Each geophone is generally deployed in a vertical orientation. Geophones having an earth spike are deployed into the earth with the earth spike downwardly disposed. Soil compaction (for buried geophones) or an inserting force (for geophones having an earth spike) are applied by a seismic technician in order to ensure favorable acoustic and seismic coupling of the geophone with the earth.
Before deploying the geophone into the earth, the seismic technician estimates the desired position (with respect to geophysical requirements) for each geophone. Each geophone is positioned by stepping off a rough distance from an adjacent geophone(s) or by roughly positioning geophone(s) in a pattern about a survey peg or other benchmark placed in or near the center of the geophone group. Each geophone is generally electronically coupled to other geophones or to a seismic data recording units.
In conventional land-based seismic studies, geophones are strung in a predetermined pattern in a geophone array across the terrain of interest. A typical geophone array pattern used in land-based seismic studies is illustrated in FIG. 1. A seismic source, such as an explosive charge, an air gun or vibroseis, is positioned within or adjacent to the geophysical spread defined by the array of geophones. Sound waves emanating from the energized seismic source into the earth are reflected and refracted back to the earth's surface by subsurface geological formations of interest. Sound waves returning to the surface are sensed by the deployed geophones that are electronically coupled to one or more seismic data recording units. Recorded sound waves, or seismic data, is processed and analyzed for use in determining formation content and properties.
Conventional land-based seismic investigations require a large number of geophones, long lengths of seismic cables, and a crew of trained seismic technicians to position and deploy the geophone array for each stage of the seismic investigation. Human error, undulations in the terrain, and natural and man-made obstacles in the terrain make it difficult to obtain accurate positions for the geophones. The accuracy of the seismic data determines the quality of the seismic analysis. In order to best determine the locations of recoverable hydrocarbon deposits, the positions of the geophones must be accurately determined. What is needed is a method of accurately determining and recording the positions of geophones deployed in a seismic array over a terrain of interest. What is needed is a method for improving the accuracy of seismic data by improving the reliability of positional data for geophones.